1. Field of the Invention
The present invention relates to a picking device for a workpiece by using a robot, in particular a picking device in which a condition for detecting an object when the object is detected by a vision sensor is utilized.
2. Description of the Related Art
In an application for picking an object or a workpiece from “a random pile”, in which a plurality of workpieces are not orderly positioned on a pallet or the like, but are disorderly stacked in a box or the like, a camera is used for widely imaging the workpieces, in order to roughly find the position of a workpiece to be picked. At this point, a detection function or a detection tool for detecting a workpiece in an image, using an image model of the workpiece, has a threshold of the size of the image of the workpiece based on a reference size of the workpiece. The detection tool detects the workpiece, the size of which is within the threshold. In the application for picking the workpiece from the random pile, in relation to the size of the image of the workpiece, there is a considerable difference between workpieces positioned at higher and lower limit positions in an area where the workpieces may exist. Therefore, the size of the image of the workpiece to be detected must have a certain range such that both workpieces positioned at higher and lower positions may be detected.
However, according to the above method, the size of the workpiece may be incorrectly detected. For example, in relation to a brake disc which is imaged as a member having some concentric circles, in pattern matching using an image model constituted by some concentric circles, the size of the model may be incorrectly detected when the difference between the sizes of the workpieces at the higher and lower limit positions is relatively large. Further, in pattern matching, as well as the workpiece detected as the concentric circles, the size may be incorrectly detected for another reason. For example, an image constituted by combining parts of a plurality of workpieces may be detected as one workpiece, depending on the stacking state of the workpieces.
In the application for picking the workpiece from the random pile, a method for determining the detection position of a vision sensor for measuring the three-dimensional position and orientation of an object is possible. In this method, an image of a workpiece positioned at a reference position is taught as “a reference size 100%”. Then, after the image is detected by a wide angle camera at a reference size of 100%, a reference detection position for a three-dimensional vision sensor is taught such that the sensor is moved and positioned at a certain distance from the workpiece. Due to this, based on information regarding the size of the workpiece at an arbitrary vertical position detected by the camera, the position of the sensor is determined such that the sensor is away from the workpiece at an arbitrary vertical position at a certain distance. However, according to this method, if the size of the workpiece is incorrectly detected, to a large extent, the vision sensor may be moved to an unsuitable position.
As a known technique for obtaining correct information regarding the three-dimensional position of a workpiece, as described in Japanese Patent Publication No. 6-61098, for example, a stereo visual device, capable of imaging an object from different points, and then detecting the position of the object by distance, based on the disparity, is used.
In a stereo measuring method as described in Japanese Patent Publication No. 6-61098, some images or vertical and horizontal edge lines obtained by imaging the object from difference points are verified, and then, reliable information regarding a cross-section of the object may be obtained. However, when the object or the workpiece is the brake disc, etc., which is relatively easy to be incorrectly detected, no workpiece may be determined to be picked if the verification of the images has failed. Further, when the verification of a workpiece located at the higher position (i.e., the workpiece is easy to be picked) fails, the picking device is directed to pick another workpiece located at the lower position. As a result, the picking device may interfere with other workpieces or external equipment, and/or, the picking device cannot pick the target workpiece due to an obstacle or another workpiece.
In order to reduce the incorrect detection of the size of the workpiece, a method using a plurality of detecting tools is possible. In this method, each detecting tool has a divided threshold calculated by dividing the range of a threshold of the size of the workpiece required for detecting all workpieces. Then, the detecting tools are appropriately switched to be used and the target workpiece is detected by means of images of the workpiece. Also in this case, however, incorrect detection may occur if the switching operation of the detection tools is not suitably carried out.
As another known method for avoiding the incorrect detection of the size of the workpiece by using information regarding the vertical position of the workpiece, a slit light scanning method for scanning a laser slit light in order to measure the vertical position of the workpiece may be possible. However, as this method uses the slit light for scanning an area where the workpiece may exist, the method is considerably disadvantageous in terms of cycle time. Further, the calculation time may increase when the method uses a detecting means for obtaining positional information of the workpiece.